Natural gas sourced offshore from underwater natural gas wells can be transported by carrier ships as an alternative to transportation using underwater pipelines. One manner of transporting natural gas is in the form of Liquid Natural Gas (LNG), and another form of transportation is Compressed Natural Gas (CNG).
With LNG, it is necessary to cool the gas down to very low temperatures in order to liquefy the gas. The gas is then re-gasified prior to delivery. Moderate pressures are involved in storing LNG in tanks on the ships during transportation, e.g. 20 bar or less. With CNG, instead the gas is simply compressed to high pressures, of the order of hundreds of bars, and transported inside suitable pressure vessels, such as steel or composite pressure vessels, on the ships. The gas is then allowed to expand at the delivery point, so that it can be delivered at a lower delivery pressure.
Existing naval CNG transportation systems and methods are known to have some disadvantages, such as a) low loading and unloading efficiencies (the loading and unloading efficiency is quantified by the ratio between the total energy or mass of CNG delivered at the delivery point by the ship, to the total energy or mass of CNG loaded at the loading point in the ship); b) risks connected with the formation of hydrates upon delivery in the delivered CNG, such as a poorer product (i.e. lower calorific value), and/or formation of ice, which is known to be able to cause structural damage to the CNG delivery equipment.